This invention relates to silicon carbide discontinuous fibers, which are substantially free of whiskers, and methods for their manufacture.
Silicon carbide whiskers are known in the art. Articles manufactured employing such whiskers are also known to be good to excellent absorbers of microwave energy, particularly low energy microwave energy, e.g., about 2.45 GHz and about 1000-3000 watts. These whiskers are commonly 1-3 microns in diameter and 10-200 microns long.
Silicon carbide whiskers, however, suffer problems relating to their potential carcinogenicity. More specifically, these whiskers, which are very small, readily inhaled, difficult to contain against dispersion into the ambient environment, among other undesirable characteristics or properties, are both difficult and expensive to manufacture and/or to be formed into a product which is useful. One proposed specific use for silicon carbide whiskers is in the fabrication of a filter for carbonaceous or organic components of a gaseous discharge stream, such as, for example, a filter for the exhaust system of a diesel engine.
Due to their small size, e.g., many times smaller than cellulose paper-making fibers which commonly are of about 7-20 microns diameter and about 50-1000 microns length, these whiskers also are unsuitable for use in the well-known and relatively inexpensive paper-making processes for forming the whiskers into a sheet, which can subsequently be formed into a pleated filter paper product, for example.
Heretofore, it has been proposed to produce silicon carbide fibers, as opposed to whiskers, to serve as heating elements in a microwave field. However, certain of these methods produce spun continuous filaments which must be cut to produce discontinuous fibers. This method is expensive and time-consuming. Moreover, cutting of the filaments tends to expose their cut ends to oxidation or other deleterious degradation. Also, it has been proposed to produce silicon carbide fibers which are somewhat free of whiskers, but the method employed requires an initial step of producing carbon fibers which are thereafter converted to silicon carbide. However, these fibers do not heat rapidly in a low energy (e.g., 2.45 MHz) microwave field. None of these known methods provides a cost-efficient and environmentally friendly means for the manufacture of substantially whisker-free silicon carbide fibers which are individually on the order of the size not materially less that the size of cellulose paper-making fibers, hence suitable for use in the manufacture of desired geometrical shapes.
The present invention comprises silicon carbide fibers of an individual size equal to or not substantially less than the size of cellulose paper-making fibers and which are substantially free of whiskers. On one embodiment, the fibers are formed employing cotton fibers, preferably chopped cotton fibers. The chopped cotton fibers are carbonized in an inert atmosphere at a temperature of between about 700xc2x0 C. and about 1200xc2x0 C. These fibers, in water, are blended with calcium oxalate monohydrate mixed in hot methanol, ferrous sulfate, and fumed silica, and thereafter dried with heating. This mix is loaded into graphite tubes and heated at an elevated temperature for a time sufficient to effect the principal reaction of:
SiO2+3Cxe2x95x90SiC+2COxe2x80x83xe2x80x83Eq.1
and resultant conversion of the fibers of the mix to silicon carbide. The process yields about 25% by weight of the original weight of carbonized cotton fibers. The vast majority of the discontinuous silicon carbide fibers are of a size approximating the size of cellulose paper-making fibers. Other components of the process product include smaller silicon fibers and/or particulates of non-fibrous geometry. This product is readily suspended in a slurry which is suitable as one of the feed materials for a slurry employed in a substantially conventional papermaking process, employing conventional and well-known paper-making equipment. In the slurry there may be included other ceramic fibers or additives, for example, as desired. The product obtained employing the paper-making process and equipment comprises a self-supporting sheet of discontinuous silicon carbide fibers which are intertangled in the manner of cellulosic fibers and additives found in a conventional paper product. The sheet product so produced has been found to be foldable, such as pleated on a pleating machine, to form a filter medium comprising principally silicon carbide fibers and, in certain circumstances, lesser quantities of entrapped silicon carbide particulates of non-fibrous geometry.
Importantly, the silicon carbide fibers of the present paper-like product obtained is strongly susceptible to relatively low-energy microwave energy and thus may be heated to a temperature of about 800 degrees C. in less than about 15 seconds. At such temperature, common organic materials entrapped in a silicon carbide fiber filter, for example, are combusted and converted to environmentally friendly products.